1. Field of the Invention
The present invention relates to a cylinder made of an aluminum alloy, adapted to be used in an internal combustion engine and having a metal plating on the inner wall surface thereof, and also to a method of treating the inner wall surface of the cylinder. In particular, the present invention relates to cylinder which is suited for use in a small air-cooled two-stroke gasoline internal combustion engine to be employed in a portable power working machine such as a brush cutter or a chain saw.
2. Description of the Related Art
As for the conventional cylinder for a small air-cooled two-stroke gasoline internal combustion engine to be employed in a portable power working machine, a typical example thereof is constructed as shown in FIG. 2. The cylinder shown in FIG. 2 is generally formed of an aluminum alloy and constituted by an integral body consisting of a barrel portion 2 having a pair of columnar expanded portions 2a disposed opposite to each other, and a head portion 3 provided with a so-called squish dome-shaped combustion chamber 4. The barrel portion 2 is provided, all over the outer wall thereof, with a large number of cooling fins 19. Further, the head portion 3 is provided with an internal thread 18 for mounting an ignition plug thereon.
The inner wall surface (cylinder bore face) 9 of the barrel portion 2 is provided with an exhaust port 5 and also with an intake port 6, which are designed to be closed and opened by the movement of the piston 20. These intake port 6 and exhaust port 5 are disposed opposite to each other in such a manner that they are offset or dislocated level-wise from each other. The columnar expanded portions 2a are respectively provided therein with a hollow scavenging passageway 7 having an inner wall 7a of predetermined thickness, each hollow scavenging passageway 7 being spaced away from these intake port 6 and exhaust port 5 by an angle of 90 degrees in the circumferential direction of the cylinder bore 9. These scavenging passageways 7 are respectively provided, at a downstream end portion (upper end portion) thereof, with a scavenging port 8 which is inclined somewhat upward, i.e. in the direction away from where the intake port 6 of the cylinder bore 9 is located. This pair of scavenging ports 8 is also designed to be opened and closed by the movement of the piston 20.
The cylinder 1 mentioned above is a so-called binary fluid scavenging type cylinder where a pair of scavenging ports 8 is symmetrically formed with respect to the longitudinal section Fxe2x80x94F taken along the middle of the exhaust port 5. However, a so-called quaternary fluid scavenging type cylinder where a pair of scavenging ports is additionally provided therewith (two pairs in total) is also known.
As for the types of the scavenging passageway, there is also known, in addition to the hollow scavenging passageway provided with an inner wall as shown in FIG. 2 and a scavenging passageway having no inner wall (the side facing the cylinder bore is opened), another kind of scavenging passageway which is provided, at a lower portion thereof, with an opening extending along the length of the scavenging passageway while leaving a half-wall at an upper portion thereof, thereby enabling the air-fuel mixture that has been introduced into the scavenging port from the crank chamber via the scavenging passageway to be contacted with the skirt portion of the piston.
The cylinder 1 made of an aluminum alloy for use in a two-stroke internal combustion engine is usually manufactured by means of a die casting method such as a high-pressure die casting method, and thereafter, the inner wall surface 9 which is adapted to be slidably contacted with a piston is usually subjected to metal plating treatment (a layer of plating 10).
Conventionally, the metal plating treatment of the inner wall surface 9 of the cylinder 1 is performed by way of iron (Fe) plating or hard chromium (Cr) plating. In particular, the surface of plating that has been obtained by way of the hard chrome plating is excellent in performance as a piston-sliding face, so that the hard chrome plating is considered indispensable for manufacturing a high-performance engine.
However, hexavalent chromium which is employed in a plating bath of the hard chromium plating method imposes a great burden on the environmental safety, and therefore, is designated as one of the special chemical substances which are considered as polluting the environment. Therefore, it is recommended all over the world to minimize the employment of hexavalent chromium. On the other hand, iron plating is minimal in burden to the environment and relatively low in production cost. Accordingly, there is a movement automakers for example to apply an iron plating to the inner wall surface of the cylinder of an internal combustion engine. However, the layer of iron plating is inferior in physical properties as compared with the layer of chromium plating. In particular, there is a great difference in hardness between them. Specifically, the Vickers hardness of the layer of iron plating which is now produced is about 500 to 600(Hv[50 gf]), which is considerably low as compared with the Vickers hardness of the layer of chromium plating which is 850 to 1,000(Hv[50 gf]). Therefore, it would be impossible to put the iron plating, as it is, into practical use in place of the hard chromium plating.
Additionally, the conventional plating treatment is poor in precision and hence the layer of plating is non-uniform, varying in thickness depending on the regions of the inner wall surface which constitutes a slide-contacting surface for the piston. As a result, it has been considered necessary to perform a finishing work such as honing work after the plating of the inner wall surface.
The present invention has been made to cope with the aforementioned circumstances, and therefore an object of the present invention is to provide a cylinder for an internal combustion engine, wherein the inner wall surface of the cylinder which constitutes a slide-contacting surface for the piston is treated with a metal plating which is minimal in burden to the environment, excellent in abrasion resistance and hard enough to endure the practical use of the cylinder, thus making it possible to dispense with a finishing work such as honing work.
Another object of the present invention is to provide a method of treating the inner wall surface of the cylinder, which is capable of obtaining an excellent cylinder as described above.
With a view to realize the aforementioned objects, there is provided, according to the present invention, a cylinder for an internal combustion engine, which is made of an aluminum alloy and is featured in that an inner wall surface of the cylinder which is adapted to be slidably contacted with a piston is electroplated in a plating bath comprising an iron plating solution containing a boron compound such as trimethylamine borane, dimethylamine borane, etc.
According to a preferable embodiment of the present invention, the layer formed on the inner wall surface by the electroplating is employed, as it is, as a finished surface without being subjected to a finishing work by means of honing, etc.
Preferably, the layer obtained by said electroplating has a Vickers hardness of 600 to 900(Hv[50 gf]). Further preferably, the deviation in thickness of the layer obtained by the electroplating all over the inner wall surface is confined within 15 xcexcm.
Further, the cylinder for an internal combustion engine according to the present invention is constructed such that a head portion constituting the combustion chamber of the engine is formed integral with a barrel portion into which the piston is slidably inserted, and is particularly suited for use in a small air-cooled two-stroke gasoline engine to be employed in a portable power working machine.
In addition, the method of treating the inner wall surface of the cylinder for an internal combustion engine according to the present invention is featured in that an inner wall surface of the cylinder which is adapted to be slidably contacted with a piston is electroplated in a plating bath comprising an iron plating solution containing a boron compound such as trimethylamine borane, dimethylamine borane, etc.
It has been confirmed that when the inner wall surface of the cylinder which is adapted to be slidably contacted with a piston is electroplated in a plating bath comprising an iron plating solution containing a boron compound such as trimethylamine borane, dimethylamine borane, etc., the hardness of the layer of plating is considerably enhanced. More specifically, while the Vickers hardness of the layer of iron plating where the aforementioned borane is not added therein is about 500 to 600(Hv[50 gf]), the Vickers hardness of the layer of iron plating where the aforementioned borane is added therein is as high as 600 to 900(Hv[50 gf]).
It has been also confirmed that the deviation in thickness of the layer of iron plating all over the inner wall surface is confined within 15 xcexcm, and that the layer of iron plating can be adhered with high precision in conformity with the dimension of cutting work.
Therefore, it is now possible, according to the present invention, to provide a cylinder for an internal combustion engine, wherein the inner wall surface of the cylinder which constitutes a slide-contacting surface for the piston is treated with an iron plating which is minimal in burden to the environment, excellent in abrasion resistance and hard enough to endure the practical use of the cylinder, thus making it possible to dispense with a finishing work such as honing work. Further, it is also possible, according to the present invention, to provide a method of treating the inner wall surface of the cylinder, which is capable of obtaining an excellent cylinder as described above.